1. Filed of the Invention
The present invention relates to a method for controlling an oxygen concentration sensor for sensing an oxygen concentration in an exhaust gas of an internal combustion engine.
2. Description of Background Information
Air/fuel ratio feedback control systems are becoming generally used for the fuel supply control of an internal combustion engine, and such systems are so constructed that the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and the air/fuel ratio of a mixture to be supplied to the engine is feedback controlled in response to a result of the detection of the oxygen concentration so as to purify the exhaust gas and to improve the fuel economy.
As an example of an oxygen concentration sensor for use in the air/fuel ratio control system of the above mentioned type, there is an oxygen concentration sensor which produces an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine in a region in which the air/fuel ratio of mixture to be supplied to the engine is larger than a stoichiometric air/fuel ratio, and the details of such a sensor are described in Japanese Patent Application laid open No. 58-153155. This oxygen concentration sensor includes a sensor element whose general construction includes a pair of flat solid electrolyte members having oxygen ion permeability. These oxygen-ion conductive solid electrolyte members are placed in the exhaust gas of the engine, and two electrodes are provided on the front and back surfaces of both of the solid electrolyte members. In other words, each pair of electrodes sandwich each solid electrolyte member. These two solid electrolyte members, each having a pair of electrodes, are arranged in parallel to each other so as to face each other and to form a gap portion, or in other words, a restricted region between them.
With this arrangement, one of the solid electrolyte members serves as an oxygen pump element and the other of the solid electrolyte members serves as a sensor cell element for sensing an oxygen concentration ratio. In the atmosphere of the exhaust gas, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrode facing the gap portion operates as a negative electrode. By the supply of this current, the oxygen component of the gas in the gap portion is ionized on the surface of the negative electrode of the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface thereof in the form of the oxygen gas.
While this movement of oxygen ions is taking place, the oxygen concentration becomes different for the gas in the gap portion and the gas outside the electrodes of the sensor cell element because of a reduction of the oxygen gas component in the gap portion. Therefore, an electric potential whose magnitude varies substantially linearly in proportion to the oxygen concentration of the gas to be measured is generated across the electrodes of the solid electrolyte member operating as the sensor cell element if the magnitude of the electric current supplied to the oxygen pump element i.e., the pump current, is constant.
By means of this electric potential generated across the electrodes of the sensor cell element, a detection as to whether the air/fuel ratio of mixture supplied to the engine is rich or lean is performed. In the case of the air/fuel ratio control system in which the air/fuel ratio is controlled by the supply of the air intake side secondary air, the secondary air is supplied when the air/fuel ratio is detected to be rich. On the other hand, the supply of the secondary air is stopped when the air/fuel ratio is detected to be lean, and the air/fuel ratio is controlled toward a target air/fuel ratio by the supply and stop of the air intake side secondary air. Further, if the magnitude of the pump current supplied to the oxygen pump element is varied so that the electric potential developing across the electrodes of the sensor cell element becomes constant, the magnitude of the pump current varies substantially in proportion to the oxygen concentration in the exhaust gas, under a condition of a constant temperature. Thus, the oxygen concentration can be detected also by the magnitude of the pump current.
In this type of oxygen concentration sensor, if an excessive current is supplied to the oxygen pump element, it causes the so called blackening phenomenon by which the oxygen ions are removed from the solid electrolyte members. For instance, when zirconium dioxide (ZrO.sub.2) is utilized as the solid electrolyte, the oxygen ions O.sub.2 are removed from the zirconium dioxide (ZrO.sub.2) so that zirconium (Zr) is separated out. As a result of this blackening phenomenon, deterioration of the oxygen pump element takes place rapidly, to cause a debasement of the operation of the oxygen concentration sensor as a whole.
In order to prevent the said phenomenon, the magnitude of the pump current supplied to the oxygen pump element must be controlled below a critical level of the occurrence of the blackening phenomenon.
In this type of oxygen concentration sensor, it is necessary that the temperature of the sensor be sufficiently higher (for example, higher than 650.degree. C.) than an exhaust gas temperature under a steady state operation, in order to obtain a proportional output signal characteristic in which the sensor output signal varies substantially in proportion to the oxygen concentration. To meet this requirement, a heating device which is made up of a heater element is incorporated in the oxygen concentration sensor and a drive current is supplied to the heater element at a time of the detection of the concentration so that heat is generated at the heater element.
Now, operating conditions of an internal combustion engine will be discussed.
When the engine load is high, the air/fuel ratio of the mixture supplied to the engine may be controlled to a rich side by the operation of a fuel increment control device of the engine such as an acceleration pump of the carburetor or a power valve. The critical value of the pump current for the occurrence of the blackening phenomenon reduces as the air/fuel ratio becomes richer. Therefore, the blackening phenomenon is likely to occur under this condition. In order to prevent the occurrence of the blackening phenomenon, the air fuel ratio of the mixture to be supplied to the engine can be controlled to the lean side while the level of the pump current is limited below the critical level of the occurrence of the blackening phenomenon. However, it was generally not possible to prevent the occurrence of the blackening phenomenon completely because there inevitably is a lag between the time that control of the air/fuel ratio of the mixture commences and the time at which a result of the air/fuel ratio control appears as a change in the oxygen concentration in the exhaust gas.
On the other hand, when the engine load is high, the amount of the mixture taken into the engine becomes also high, and this results in an increase of the combustion temperature. Under such a condition, the temperature of the exhaust gas also rises to a level higher than the temperature of the heater element. Because of this, there is a possibility of rapid deterioration of the heater element. The assignee of the present application has proposed a control method in which the supply of the heater current to the heater element is stopped so as to prevent a rapid deterioration of the heater element when the engine load is high. However, in the case of such a control method, a time period is required for the resumption of the operation of the pump element and the sensor cell element if the supply of the heater current to the heater element is always stopped under a high load condition. Therefore, an accurate sensing of the air/fuel ratio using the output signal level of the oxygen concentration sensor is not possible even if the condition of the feedback control is satisfied.
Further, if the high load operation of the engine is detected by means of the rotational speed of the engine, the supply of the heater current to the heater element is enabled or stopped repeatedly when the engine speed is at around a reference speed for the detection of the high load operation. Above all, the repetition of the supply or the stop of the heater current may shorten the life of the heater element. Further, under such a condition, the output signal level of the oxygen concentration sensor fluctuates even though the air/fuel ratio is constant, because of the change in the calorific power of the heater element. For avoiding this problem, it is conceivable to set the reference speed for detecting the high load operation at a low level. However, such a method is not desirable because the feedback control range of the air/fuel ratio is reduced to lower the performance of the emission control.
Moreover, once the supply of the heater current is stopped, it requires a time period after a resumption of the supply of the heater current until the oxygen pump element and the sensor cell elements are activated again. Therefore, it is not possible to detect the air/fuel ratio accurately from the output signal level of the oxygen concentration sensor when the engine speed is reduced below the reference speed, to satisfy the condition of the air/fuel ratio feedback control. Because of the reasons described above, repetition of the supply and stopping of the heater current to the heater element leads to a reduction of the range of the air/fuel ratio feedback control.